The invention relates to a colour display tube comprising in an evacuated envelope means to generate a number of electron beams, a display screen having areas luminescing in different colours, and a colour selection electrode situated near the display screen and having apertures for passing through the electron beams and associating each electron beam with luminescent areas of a respective colour. The colour selection electrode is coated on at least the side remote from the display screen with the layer of a material comprising a heavy metal having an atomic number exceeding 70.
U.S. Pat. No. 3,562,518 discloses a colour display tube in which the colour selection electrode has a layer containing at least 20 mg of bismutch oxide per cm.sup.2. The object of this layer is to reduce the quantity of X-ray radiation which is passed through the color selection electrode to the rear side of the tube after the radiation is generated by high-energy electrons impinging on the display screen.
During operation of a colour display tube having a colour selection electrode, usually called a shadow mask, only a small part of each electron beam is passed through the apertures of the shadow mask. Approximately 80 percent of the electrons are intercepted by the shadow mask on their way to the display screen. The kinetic energy of the electrons impinging on the shadow mask is converted for the greater part into thermal energy so that the temperature of the mask increases and hence the shadow mask experiences thermal expansion. Since the shadow mask is usually connected in a rigid supporting frame, the temperature of the shadow mask during warm-up will rise more rapidly in the centre than at the edge. The thermal expansion of the shadow mask associated with the rise in temperature results overall doming of the mesh in the direction towards the display screen. Furthermore, when a large quantity of electrons impinges on a location on the shadow mask, localized doming of the shadow mask will occur if temperature equilibrium in the plane of the shadow mask does not take place sufficiently rapidly. Both the local doming and the overall doming of the shadow mask results in a displacement of the spot formed on the display screen by the electrons passing through the mask apertures and colour defects occur in the picture displayed on the display screen.
In connection with this problem it is known from Japanese Patent Application No. 55.76553 to provide an electron-reflecting layer on the colour selection electrode, which layer also comprises a heavy metal, for example bismuth, lead or tungsten. The layer has a thickness of approximatley 10 microns and prevents the electrons incident on the colour selection electrode from penetrating into the colour selection electrode and converting their kinetic energy into thermal energy.
It has been found, however, that by using such a layer a number of detrimental side effects may occur. Notably, due to the large electron reflection power of the layer and the increased thickness of the colour selection electrode resulting from provision of the layer, an increased reflection of the electrons occurs at the walls of the apertures in the colour selection electrode. These reflected electrons impinge on the display screen in arbitrary places and deteriorate the picture quality. Also, as the layer thickness increases, the possibility of the formation of loose particles in the tube also increases. These loose particles may, inter alia, lead to high voltage flash-overs in the electron gun and to black spots in the picture displayed on the screen. Furthermore, by providing thick layers the sizes of the apertures in the colour selection electrode transmission might be reduced, thereby decreasing through the colour selection electrode.